The main advantage of heel & toe shifting is that you can brake as well as rev match at the same time thus saving you essential time.
What does heel and toe do?
1. Reduces stress on the driveline
2. Prevents forward weight transfers when down shifting
3. Reduces the chances of wheel lock in extreme cases
4. Allows rapid, smooth down changes before entering a corner
How does heel n toe work?
Basically you use your right foot to brake and accelerate simultaneously while your left foot does the duty on the clutch pedal.
Here is a video to demonstrate clearly how it is done.
So while you are slowing down the car before entering a corner you are also accelerating at the same time to be able to downshift and keep the engine in the powerband for the lower gear.